The extent of adsorption of phosphate onto pre-polymerized hydrous ferric oxides (HFO) is strongly dependent upon pH, HFO age and concentration of competing anions with the phosphate adsorption capacity decreasing significantly with increase in pH, HFO age and concentration of competing anion. A semi-empirical diffuse layer model and a molecular model (the CD-MUSIC model), which take these factors into account, have been developed and provide good description of the phosphate adsorption behavior over a wide range of conditions. Although there are significant differences between these two modeling approaches, both models yield similar results with regard to the extent of reduction in HFO specific surface area and, concomitantly, phosphate removal on ageing of the HFO. While the method of accounting for binding of citrate to HFO surface sites differs somewhat between the models, both approaches provide a good description of the ability of this competing anion to prevent phosphate adsorption and removal from solution.